Studies on Selected Microbial Biotransformations of Arsenic Compounds
Arsenic is an element with a notorious history and reputation used both as a poison and a cure in its past. It has a complex biogeochemical cycle in nature which was investigated. The major portion of this project looked primarily at arsenobetaine. Arsenobetaine is a benign form of arsenic that is ubiquitous throughout the marine environment representing the major form of arsenical found in higher marine organisms. It is from this that a great deal of interest stems, as the human population has tried to reassure itself on the safety of seafood. The biodegradation of arsenobetaine is described, in specific its breakdown by mixed biota with analysis by HPLC-hydride generation-atomic fluorescence spectroscopy (HG-AFS). Subsequent selection within this consortium led us to four bacterial isolates that demonstrated the biodegradation of arsenobetaine to dimethylarsinate. Two of these isolates are identified to species level {Paenibacillus glucanolyticus and Pseudomonas fluorescens A). With analysis by HPLC-inductively coupled plasma-mass spectrometry (ICP-MS), the degradation of arsenobetaine is described by initial cleavage of the methyl arsenic bond to form dimethylarsinoylacetate, with subsequent cleavage of the carboxymethyl-arsenic bond to yield dimethylarsinate. It has been suggested that mixed community function would provide a means for the biodegradation of arsenobetaine, with different communities being capable of breaking the two different types of carbon-arsenic bonds. The present work indicates this is not an obligate requirement and evidence of the monoseptic culture degradation of arsenobetaine has been described. The biogenesis of arsenobetaine, a previously undescribed process, was also investigated. A lysed cell extract of Pseudomonas fluorescens A NCIMB 13944 was shown to transform dimethylarsinoylacetate to arsenobetaine and dimethylarsinate. Dimethylarsinoylacetate is seen as a likely intermediate in the biosynthesis pathway starting at dimethylarsinoyiriboside sugars. dimethylarsinoylethanol and on to arsenobetaine. The present work indicates that in this proposed biosynthetic pathway, oxidation would precede the reduction and methylation at the arsenic atom indicating that dimethylarsinoylacetate is the intermediate prior to arsenobetaine as opposed to arsenocholine. It is also demonstrated that S-adenosyl methionine (SAM) markedly promotes the bioconversion of dimethylarsinoylacetate to arsenobetaine suggesting that the bioconversion is catalysed by a methyltransferase enzyme using SAM as methyl donor. Finally, the research focused on the solubilisation of arsenic, examining arsenate and arsenite biotransformation within soils. Looking ultimately at bacteria capable of using arsenate as sole terminal electron acceptor in order to achieve growth by dissimilatory arsenate reduction. Strains capable of dissimilatory arsenate reduction were isolated and identified. The ability to microbiologically increase solubilisation of arsenic, rendering it into a more mobile form, within the environment is described. This could make it a viable technique for use in detoxifying arsenic contaminated sites.
Networked Digital Library of Theses and Dissertations